A printed Sequence Listing accompanies this application, and has also been submitted with identical contents in the form of a computer-readable ASCII file on a floppy diskette.
1. Field of the Invention
The present invention is broadly concerned with a method for designing single copy hybridization probes useful in the fields of cytogenetics and molecular genetics for determining the presence of specific nucleic acid sequences in a sample of eukaryotic origin, e.g., the probes may be used to analyze specific chromosomal locations by in situ hybridization as a detection of acquired or inherited genetic diseases. More particularly, the invention pertains to such probes, hybridization methods of use thereof and techniques for developing the probes, where the probes are essentially free of genomic repeat sequences, thereby eliminating the need for disabling of repetitive sequences which is required with conventional probes.
2. Description of the Prior Art
Chromosome abnormalities are associated with various genetic disorders, which may be inherited or acquired. These abnormalities are of three general types, extra or missing individual chromosomes (aneuploidy), extra or missing portions of chromosomes (including deletions, duplications, supernumerary and marker chromosomes), or chromosomal rearrangements. The latter category includes translocations (transfer of a piece from one chromosome onto another chromosome), inversions (reversal in polarity of a chromosomal segment), insertions (transfer of a piece from one chromosome into another chromosome) and isochromosomes (chromosome arms derived from identical chromosomal segments). The abnormalities may be present only in a subset of cells (mosaicism), or in all cells. Inherited or constitutional abnormalities of various types occur with a frequency of about one in every 250 human births, with results which may be essentially benign, serious or even lethal. Chromosomal abnormalities are common and often diagnostic in acquired disorders such as leukemia and other cancers.
Hybridization probes have been developed in the past for chromosome analysis and diagnosis of abnormalities. The probes comprise clone or amplified genomic sequences or cDNA. For example, U.S. Pat. Nos. 5,447,841, 5,663,319 and 5,756,696 describe hybridization probes in the form of labeled nucleic acids which are complementary to nucleic acid segments within target chromosomal DNA. However, these probes contain repetitive sequences and therefore must be used in conjunction with blocking nucleic acids which are substantially complementary to repetitive sequences in the labeled probes. That is, these prior art probes are either pre-reacted with the blocking nucleic acids so as to bind and block the repetitive sequences therein, or such blocking nucleic acids are present in the hybridization reaction mixture. If the repetitive sequences in the probes are not disabled in some manner, the probes will react with the multiple locations in the target chromosomal DNA where the repetitive sequences reside and will not specifically react with the single copy target sequences. This problem is particularly acute with interspersed repeat sequences which are widely scattered throughout the genome, but also is present with tandem repeats clustered or contiguous on the DNA molecule. The requirement for repeat sequence disabilization by using complementary blocking nucleic acids reduces the sensitivity of the existing probes. Reliable, easily detectable signals require DNA probes of from about 40-100 kb.
The prior art also teaches that cloned probes presumed to contain single copy sequences can be identified based on their lack of hybridization to genomic DNA. In these other studies, hybridization is first performed with probes that contain pools of clones in which each recombinant DNA clone has been individually selected so that it hybridizes to single-copy sequences or very low copy repetitive sequences. A prerequisite step in this prior art is to identify single copy sequences by experimental hybridization of labeled genomic DNA to a candidate DNA probe by Southern or dot-blot hybridization. Positive hybridization with labeled total genomic DNA usually indicates that the candidate DNA probe contains a repetitive sequence and eliminates it from consideration as a single copy probe. Furthermore, an experimental hybridization of a DNA probe with total genomic DNA may fail to reveal the presence of multicopy repetitive sequences that are not abundant ( less than 100 copies) or are infrequent in the genome. Such sequences represent a small fraction of the labeled genomic DNA and the signal they contribute will be below the limits of detection.
It has also been suggested to physically remove repeat sequences from probes by experimental procedures (Craig et al., Hum. Genet., 100:472476 (1997); Durm et al., Biotech., 24:820-825 (1998)). This procedure involves prehybridizing a polymerase chain reaction (PCR)-amplified genomic probe with an excess of purified repetitive sequence DNA prior to applying the probe to the DNA target. The resulting purified probe is depleted of repetitive sequences. This procedure is in principle very similar to other procedures that disable the hybridization of repetitive sequences in probes, but the technique is time-consuming and does not provide any advantages over the probes described in U.S. Pat. Nos. 5,447,841 and 5,756,696.
The present invention overcomes the problem outlined above and provides nucleic acid (e.g., DNA) hybridization probes comprising a labeled, single copy nucleic acid which hybridizes with a deduced single copy sequence interval in target nucleic acid of known sequence. Generally speaking, the probes of the invention are designed by comparing the sequence of a target nucleic acid with known repeat sequences in the genome of which the target is a part; with this information it is possible to deduce the single copy sequences within the target (i.e., those sequences which are essentially free of repeat sequences which, due to the lack of specificity, can mask the hybridization signal of the single copy sequences). As can be appreciated, these initial steps require knowledge of the sequences both of the target and genomic repeats, information which is increasingly available owing to the Human Genome Project and related bioinformatic studies. Furthermore, readily available computer software is used to derive the necessary single copy sequences.
The probes hereof are most preferably complementary to the target sequence, i.e., there is a 100% complementary match between the probe nucleotides and the target sequence. More broadly, less than 100% correspondence probes can be used, so long as the probes adequately hybridize to the target sequence, e.g., there should be at least about 80% sequence identity between the probe and a sequence which is a complement to target sequences, more preferably at least about 90% sequence identity.
Nucleic acid fragments corresponding to the deduced single copy sequences can be generated by a variety of methods, such as PCR amplification, restriction or exonuclease digestion of purified genomic fragments, or direct nucleic acid synthesis. The single copy fragments are then purified to remove any potentially contaminating repeat sequences, such as, for example, by electrophoresis or denaturing high pressure liquid chromatography; this is highly desirable because it eliminates spurious hybridization and detection of unrelated genomic sequences.
The probe fragments may then be cloned into a recombinant DNA vector or directly labeled. The probe is preferably labeled by nick translation using a modified or directly labeled nucleotide. The labeled probe is then denatured and hybridized, preferably to fixed chromosomal preparations on microscope slides or alternately to purified nucleic acid immobilized on a filter, slide, DNA chip, or other substrate. The probes can then be hybridized to chromosomes according to conventional fluorescence in situ hybridization (FISH) methods such as those described in U.S. Pat. Nos. 5,985,549 or 5,447,841; alternately, they can be hybridized to immobilized nucleic acids according to the techniques described in U.S. Pat. Nos. 5,110,920 or 5,273,881. Probe signals may be visualized by any of a variety of methods, such as those employing fluorescent, immunological or enzymatic detection reagents.